Method of producing a hot rolled and age hardened columbium-bearing steel product

ABSTRACT

A high strength low alloy steel product consisting essentially of 0.06 to 0.14 percent carbon, 0.6 to 1.5 percent manganese, 0.02 percent maximum silicon, 0.07 percent maximum phosphorous, 0.04 to 0.12 percent columbium, balance iron is produced in the hot rolled and aged condition by hot rolling at a finishing temperature of 1550* to 1650*F, collecting the steel at a temperature of 1000* to 1250*F and age hardening by continuous heat treatment at a temperature of 1050* to 1250*F for 0.5 to 5 minutes. The above treated product may be further subjected to galvanizing.

United States Patent. [191 Bucher et al.

[451 May 27, 1975 [75] Inventors: John H. Bucher; Daniel F. Jancula,

both of Pittsburgh, Pa.

[73] Assignee: Jones & Laughlin Steel Corporation,

Pittsburgh, Pa.

[22] Filed: June 13, 1974 [21] Appl. No.: 479,007

[52] U.S. Cl. l48/l2.3 [51] Int. Cl C2ld 7/14 [58] Field of Search148/12 F, 12.3

[56] References Cited UNITED STATES PATENTS 3,666,570 5/1972 Korchynskyet a1. 148/12.3 3,761,324 9/1973 Elias et al. 148/36 3,765,874 10/1973Elias et a1. 148/36 3,849,209 11/1974 lshizaki et al. 148/12 F PrimaryExaminer-W. Stallard Attorney, Agent, or FirmGerald K. White; T. A.Zalenski [5 7] ABSTRACT A high strength low alloy steel productconsisting essentially of 0.06 to 0.14 percent carbon, 0.6 to 1.5percent manganese, 0.02 percent maximum silicon, 0.07 percent maximumphosphorous, 0.04 to 0.12 percent columbium, balance iron is produced inthe hot rolled and aged condition by hot rolling at a finishingtemperature of l550 to 1650F, collecting the steel at a temperature of1000 to 1250F and age hardening by continuous heat treatment at atemperature of 1050 to 1250F for 0.5 to 5 minutes. The above treatedproduct may be further subjected to galvanizmg.

6 Claims, No Drawings METHOD OF PRODUCING A HOT ROLLED AND AGE HARDENEDCOLUMBIUM-BEARING STEEL PRODUCT Our invention is generally related tothe production of columbium-bearing high strength low alloy steelproducts in the hot rolled, age hardened metallurgical state and havinga minimum yield strength of 80,000 p.s.i. The process generallycomprises subjecting a columbium-containing, low carbon steel to athermomechanical practice which produces a fine grained, hot-rolledsteel product in which incomplete precipitation of columbium carbideshas occurred. Yield strengths on the order of 70,000 to 80,000 p.s.i. orgreater are obtained at this stage. The hot-rolled strip is thereaftercontinuously heat-treated in order to cause further precipitation ofcolumbium carbides and to thus obtain a minimum yield strength of 80,000p.s.i. Such precipitation of columbium carbides or age hardening servesto increase the yield strength by about 3,000 to 15,000 p.s.i. Followingaging, the hot-rolled and age hardened product may be galvanized in acontinuous manner.

Columbium-bearing steel products are the subject of much interest andintensive effort in the art due to their desirable combination of highstrength and relatively low cost. Typical of such activity are US. Pat.Nos. 3,010,822, 3,102,831, 3,619,303, 3,666,570, and 3,671,334 as wellas an article appearing in Metallurgical Society of Transactions of theAIME, Volume 239, April, 1967, pages 478-485.

United States Patent Numbers 3,010,822, 3,102,831 and 3,666,590 relateto the controlled thermomechanical processing of columbium-bearingsteels but do not, however, relate to an aging or precipitationheat-treatment to enhance yield strength levels. Moreover, one followingthe general teachings of US. Pat. No. 3,619,303 would be led to believethat an age hardening heat-treatment would be ineffective upon theproduct produced in accordance with the above three patents because ofthe lack of molybdenum in the respective steel compositions. US. Pat.No. 3,619,303 discloses a ferrite-carbide, low alloy,-constructionalsteel which is age hardenable. This patent teaches thatcolumbium-bearing steels of the composition described therein will notexhibit any significant change in mechanical properties upon agingunless molybdenum is present. United States Patent Number 3,671,334 isdirected to strainaged articles manufactured from renitrogenized,columbium-bearing, high-strength steels. The hot-rolled product isstrained and aged to develop yield strengths in the vicinity of 70,000to 90,000 p.s.i.-

Age hardening is due to the formation of iron carbides and nitrides.Columbium functions to increase strength primarily due to its role as agrain refining agent rather than as a precipitation hardening agent. Theabove mentioned AIME articles discloses solution treating and agingcolumbium-bearing ferritic steel.

The invention is based upon the discovery that columbium-bearing highstrength steels can be age hardened to yield strength levels in excessof 80,000 p.s.i. without resort to costly alloying additions such asmolybdenum through use of balanced carbon and columbium contents incombination with a controlled thermo-mechanical procedure and continuousage hardening treatment. Such procedure is advantageous in that theincompletely hardened hot rolled product can be conveniently agehardened by passage through heat-treating equipment that is locatedin-line with galvanizing facilities. Hence. one may advantageouslyemploy existing equipment to perform the dual functions or age hardeningand zinc coating.

It is thus an object of our invention to provide a process that iscapable of producing an age hardened columbium-bearing steel producthaving a yield strength in excess of 80,000 p.s.i. and characterized bythe absence of costly alloying ingredients.

It is a further object of our invention to produce a columbium-bearinggalvanized steel product in the age hardened condition and having ayield strength in excess of 80,000 p.s.i.

These and other objectives and advantages of our invention will beapparent to those skilled in the art from the following description ofthe invention.

In order to produce an incompletely hardened or aged columbium-bearinghot rolled steel product and then to subsequently age harden suchproduct to yield strength levels greater than about 80,000 p.s.i.,careful control of chemical composition, thermo-mechanical processing,and heat-treatment is required. All of the above mentioned processinglimits must be controlled to fall within predetermined limits in orderto achieve a product having the desired mechanical properties.

Steel suitable for use in the invention may be manufactured by anyconventional steelmaking process. Typical processes include the openhearth, basic oxygen, and electric furnace processes. Followingsteelmaking, molten steel is cast into conventional intermediateproducts such as ingots, billets, or slabs and then ultimately subjectedto hot rolling into a hot rolled product such as strip.

Steel having the chemical composition shown in Table I is suitable foruse in the practice of the invention.

In addition zirconium, titanium, or rare earths may be optionallypresent in an amount sufficient to form globular sulfides which will notbecome elongated during hot-rolling. This procedure enhances bendabilityand formability of the final product. Sulfide shape control withzirconium or rare earth additions is taught in US. Pat. No. 3,666,570.Further discussion of the role of zirconium and rare earths as well astitanium may be found in the article, Control of Sulfide Shape in LowCarbon Al-Killed Steel, E. J. Licky et al, Journal of Metals, July,1965, pages 769-775.

Aluminum is present in an amount sufficient to produce a killed steelproduct. Typical aluminum concentrations for such purpose range from aminimum of 0.02 percent.

Nitrogen is present in residual amounts and is not purposefully added tothe steels of the invention. It will be understood by those skilled inthe art that residual nitrogen content may vary with the type ofsteelmaking process. As an example, steel produced by the open hearthprocess typically contains residual nitrogen in amounts on the order of0.002 to 0.006 percent.

Phosphorous may be present in amounts up to 0.07 percent. Higher valuesshould be avoided due to em brittling effects Although some degree ofminimal strengthening may occur due to the formation of iron nitridesor, in the event that zirconium is employed as a sulfide shape controlagent, zirconium nitrides, the primary secondary phase strengtheningmechanism involves the precipitation of columbium carbides. A majorfactor of the invention resides in the selection of the columbium andcarbon contents of the steel. The choice of the respective contents,along with the later described thermo-mechanical processing sequence,results in the incomplete formation or precipitation of columbiumcarbides in the hot-rolled condition. This attainment of thismetallurgical state is a prerequisite for the subsequent yield strengthincrease obtained through age hardening. In order to obtain incompletecolumbium carbide precipitation in the hot-rolled state, a relativelyhigh columbium content and relatively low carbon content is required.The terms relatively high" and relatively low" are used in the contextof the general ranges taught in US. Pat. No. 3,666,570 which discloses ageneral columbium content range of0.01 to 0.08 percent and a generalcarbon content range of 0.06 to 0.20 percent. For purposes of ourinvention columbium and carbon should be generally restricted to 0.04 to0.12 percent and 0.06 to 0.14 percent, respectively. As may be observed,columbium is maintained at a relatively high level and carbon at arelatively low level. If one were to employ relatively high columbiumand carbon levels or relatively low columbium and relatively high carbonlevels, complete precipitation of columbium carbides would occur and,hence, the hot-rolled product would not be age hardenable. On the otherhand, relatively low columbium and carbon contents are not capable ofproducing a sufficient degree of age hardening to achieve yield strengthlevels on the order of 80,000 psi. By employing the relatively highcolumbium and relatively low carbon levels of the invention, an ageablehot-rolled product capable of attaining a yield strength in excess of80,000 may be obtained. Columbium and carbon contents on the order of0.05 to 0.10 percent and 0.08 to 0.12 percent, respectively arepreferred as such relatively finer control of ingredients tends topermit a greater degree of control of columbium carbide precipitation.An additional advantage associated with lower carbon levels is that ofenhanced weldability.

Manganese in amounts of from about 0.6 to 1.5 percent promotes theachievement of a basic strength level to which secondary strengtheningdue to columbium carbide precipitation is additive. The preferredmanganese range is about 1.0 to 1.2 percent and is selected so as to begenerally toward the high side of the general range in order to furtherensure that yield strength values in excess of 80,000 psi. are attained.

Silicon is maintained at a level of 0.02 percent maximum for purposes ofoptimizing zinc adherence during galvanizing.

Despite the teachings of US. Pat. No. 3,619,303 that molybdenum inamounts of from 0.04 to 0.50 percent is an indispensable alloyingelement if one wishes to age harden columbium-bearing steels, we havefound that if one utilizes the described chemical composition andthermomechanical practice, significant age hardening of the hot-rolledproduct may be obtained. Thus, one

of the major advantages of the invention is related to the ability toachieve a yield strength increase on the order of 3,000 to 15,000 p.s.i.through an age harden- 5 ing treatment. This unexpected abilityobviously leads to cost reductions that are of significance in theprocessing of large volumes of steel. The steel of the inventioncontains molybdenum only in residual quantities, i.e., no intentionalmolybdenum additions are made to the steel. Typically such levels are nomore than about 0.01 or 0.02 percent. Residual levels are a function ofscrap mix and other raw material input factors and consequently may varysomewhat in given instances. Generally, however, residual molybdenumdoes not exceed about 0.02 percent.

Having selected a steel composition that is capable of being transformedinto a hot-rolled product characterized by the incomplete formation ofcolumbium carbides, the cast or intermediate product is then subjectedto hot rolling according to prescribed finishing and collectiontemperature ranges. Such processing parameters are necessary in order toachieve the desired precipitation state of the invention as well as afine grain size. Fine grain sizes, typically on the order of ASTM No. 13to ASTM No. 15, also promote high yield strength levels.

Hot-rolling should be performed with finishing temperatures on the orderof about l550 to 1650F. Collection of the hot-rolled steel product isaccomplished by either coiling or piling. Collection temperatures on theorder of from l000 to 1250F are suitable for purposes of the invention.At this stage of manufacture, the steel typically has a yield strengthof about 70,000 to about 80,000 p.s.i. or slightly higher.

Following hot-rolling, collecting, and, optionally, pickling, the steelis subjected to a continuous age hardening treatment which serves topromote further columbium carbide formation and to increase the yieldstrength by about 3,000 to 15,000 p.s.i. Continuous heat treatment at atemperature of from about l050 to 1250F for a time of from about 0.5 to5 minutes is sufficient to attain the desired strength increase andthereby consistently produce an 80,000 p.s.i. minimum yield strengthproduct having acceptable ductility and toughness. The choice of timesand temperatures within those described above is somewhat dependent uponline speed and product thickness.

Suitable apparatus for conducting the age hardening step is a continuousannealing furnace which commonly precedes the galvanizing step in mostcommercial galvanizing processes for flat rolled steel products.Normally such annealing furnaces are operated at a temperature range ofl500 to 1600F for travel time of 0.5 to 5 minutes. Such procedure wouldnot result in the desired age hardening for the steel of the invention.Hence, it is necessary to operate the furnace at lower than normaltemperatures. It is especially advantageous to employ the annealingfurnace unit of a continuous galvanizing line to the product of theinvention whenever it is also desired to zinc coat the steel due to thefact that the respective treatments are in-line with each other and theaged and zinc coated product is characterized by a commerciallyattractive combination of strength and ductility.

An example which demonstrates a specific embodiment of the invention isas follows. A steel having a composition of0. 12 percent carbon, 0.91percent manganese, 0.052 percent phosphorous, 0.025 percent sulb.collecting said hot rolled steel at a temperature of fur, 0.053 percentcolumbium, 0.10 percent zirconium, from l000 to l250F; and 0.03 percentmolybdenum, and 0.06 percent aluminum 0. age hardening said hot rolledsteel by continuously was cast and hot-rolled into a slab usingconventional heat treating the steel at a temperature of from practices.The slab was then hot-rolled into a 0.075 5 1050 t 125()F for a time offrom 0.5 to 5 minutes inch thick Coll at a finishing temperature of15800}: and so as to increase the yield strength of said hot rolled acoiling temperature of 1220F. The as-rolled yield Steel by f about 3 000to 15 000 i strength of the hot-rolled product was 84,000 p.s.i.

Following pickling to prepare the coil surface for gal- A process forproducing 21 killed high strength low vanizing, the hot-rolled coil wasthen pr ce sed 10 alloy steel product having a yield strength in excessof through a conventional continuous annealing furnace about 80,000 psi.in the hot rolled and age hardened to effect aging and then through agalvanizing bath to condition according to claim 1, wherein:

produce a galvanized product. Furnace temperature the steel consistsessentially of 0.08 to 0.12 percent was maintained at ll70F for thefront portion of the carbon, 1.0 to 1.2 percent manganese, and 0.05 tocoil, ll90F for the middle portion of the coil, and 0,10 percentcolumbium.

ll50F for the back portion of the coil. Furnace time was on the order ofthree minutes. A 1% 02. minimum 3- A process f0! producing 3 killed highstrength low spangle zinc coating was applied to both sides of the alloysteel product having a yield strength in excess of coil. The galvanizedcoil was then split into two coils, about 80,000 psi. in the hot rolledand age hardened A and B, for purposes of evaluation. Coils A and Brepcondition according to claim 1, wherein:

resented the front and back portions of the original the steel consistsessentially of .02% maximum mocoil, respectively. Coil A was tested atits front portion lybdenum. so as to be representative of the ll70Ftemperature. I Coil B was tested at both ends so as to be representativeA Process for prodflcmg kllled hlgh S g low of temperatures of 1190F andb ll50F. alloy steel product having a yield strength in excess of Themechanical properties shown in Table II were aboul 80,000 in the hrolled i age hardened obtained from the two test coil portions: Condmonaccording to Clalm 9 Wheremi TABLE II Furnace Yieldtimate Total Temp.Strength Strength Elongation Hardness Coil (F) Orientation (k.s.i.)(k.s.i.) (70) (R,,)

A ll70 Longitudinal 87.9 94.0 20 92 Transverse 86.8 94.7 20 B ll90Longitudinal 91.8 97.4 18 95 Transverse 91.8 97.0 18 B l 150Longitudinal 84.5 92.1 20 92 Transverse 88.4 95.4 20

A may b b d f the above table, h various said steel further consistsessentially of a sulfide inage hardening temperatures were sufficient toproduce Clusion Shape COIltfOl ag nt Selected from the an increasedyield strength level for a steel having the gr p isting of Zirconium,titanium, and rare composition of and processed in accordance with theearths in an mount Sufficient to form globular sulinventi0n tides in thehot rolled and age hardened condition.

5. A process for producing a killed high strength low We claim: alloysteel product having a yield strength in excess of I about 80,000 psi.in the hot rolled and age hardened 1. A process for produclng a il gStrength 10W condition according to claim 1, which further includes:

alloy steel product having a strength in excess Of passing said hotrolled and age hardened steel prodabout 80,000 psi. in the hot rolledand age hardened ct through a molten Zinc bath whereby Said hotcondition, comprising: rolled and age hardened steel product is coatedwith zinc.

6. A process for producing a killed high strength low alloy steelproduct having a yield strength in excess of about 80,000 psi. in thehot rolled and age hardened condition according to claim 5, wherein:

a. hot rolling a steel consisting essentially of 0.06 to 0.14 percentcarbon, 0.6 to 1.5 percent manganese, 0.07 percent maximum, phosphorous,0.04 to 0.12 percent columbium, molybdenum in residual quantities,nitrogen in residual quantities, balance the Steel Consists essentiallyof a maximum of O 02 iron, at a finishing temperature of from l550 topercent Silicon. 165w. r. r

1. A PROCESS FOR PRODUCING A KILLED HIGH STRENGTH LOW ALLOY STEELPRODUCT HAVING A YIELD STRENGTH IN EXCESS OF ABOUT 80,000 P.S.I. IN THEHOT ROLLED AND AGE HARDENED CONDITION, COMPRISING: A. HOT ROLLING ASTEEL CONSISTING ESSENTIALLY OF 0.06 TO 0.14 PERCENT CARBON, 0.6 TO 1.5PERCENT MANGANESE, 0.07 PERCENT MAXIMUM, PHOSPHOROUS, 0.04 TO 0.12PERCENT COLUMBIUM, MOLYBDENUM IN RESIDUAL QUANTITIES, NITROGEN INRESIDUAL QUANTITIES BALANCE IRON, AT A FINISHING TEMPERATURE OF FROM1550* TO 1650*F, B. COLLECTING SAID HOT ROLLED STEEL AT A TEMPERATUE OFFROM 1000* TO 125*F, AND C. AGE HARDENING SAID HOT ROLLED STEEL BYCONTINOUSLY HEAT TREATING THE STEEL AT A TEMPERATURE OF FROM 1050* TO1250*F FOR A TIME OF FROM 0.5 TO 5 MINUTES SO AS TO INCREASE THE YIELDSTRENGTH OF SAID HOT ROLLED STEEL BY FROM ABOUT 3,000 P.S.I.
 2. Aprocess for producing a killed high strength low alloy steel producthaving a yield strength in excess of about 80,000 p.s.i. in the hotrolled and age hardened condition according to claim 1, wherein: thesteel consists essentially of 0.08 to 0.12 percent carbon, 1.0 to 1.2percent manganese, and 0.05 to 0.10 percent columbium.
 3. A process forproducing a killed high strength low alloy steel product having a yieldstrength in excess of about 80,000 p.s.i. in the hot rolled and agehardened condition according to claim 1, wherein: the steel consistsessentially of .02% maximum molybdenum.
 4. A process for producing akilled high strength low alloy steel product having a yield strength inexcess of about 80,000 p.s.i. in the hot rolled and age hardenedcondition according to claim 1, wherein: said steel further consistsessentially of a sulfide inclusion shape control agent selected from thegroup consisting of zirconium, titanium, and rare earths in an amountsufficient to form globular sulfides in the hot rolled and age hardenedcondition.
 5. A process for producing a killed high strength low alloysteel product having a yield strength in excess of about 80,000 p.s.i.in the hot rolled and age hardened condition according to claim 1, whichfurther includes: passing said hot rolled and age hardened steel productthrough a molten zinc bath whereby said hot rolled and age hardenedsteel product is coated with zinc.
 6. A process for producing a killedhigh strength low alloy steel product having a yield strength in excessof about 80,000 p.s.i. in the hot rolled and age hardened conditionaccording to claim 5, wherein: the steel consists essentially of amaximum of 0.02 percent silicon.